This disclosure relates to dovetail geometry for fan blades used in individually bladed rotors of gas turbine engines.
A gas turbine engine typically includes a fan section, a compressor section, a combustor section and a turbine section. Air entering the compressor section is compressed and delivered into the combustion section where it is mixed with fuel and ignited to generate a high-speed exhaust gas flow. The high-speed exhaust gas flow expands through the turbine section to drive the compressor and the fan section. The compressor section typically includes low and high pressure compressors, and the turbine section includes low and high pressure turbines.
The high pressure turbine drives the high pressure compressor through an outer shaft to form a high spool, and the low pressure turbine drives the low pressure compressor through an inner shaft to form a low spool. The fan section may also be driven by the low inner shaft. A direct drive gas turbine engine includes a fan section driven by the low spool such that the low pressure compressor, low pressure turbine and fan section rotate at a common speed in a common direction.
A speed reduction device such as an epicyclical gear assembly may be utilized to drive the fan section such that the fan section may rotate at a speed different than the turbine section so as to increase the overall propulsive efficiency of the engine. In such engine architectures, a shaft driven by one of the turbine sections provides an input to the epicyclical gear assembly that drives the fan section at a reduced speed such that both the turbine section and the fan section can rotate at closer to optimal speeds.
Individual fan blades are mounted within a hub or rotor driven by the gear assembly. The configuration and geometry of the fan blades balance propulsive efficiency with durability and fatigue requirements.
A dovetail of each fan blade is received in a correspondingly shaped slot in the fan rotor. The dovetail provides a bearing surface which reacts against a load surface of the slot. It is desirable to provide a small dovetail for weight savings. However, the dovetail is sized to provide sufficient strength to retain the fan blades in the fan rotor throughout engine operation and during a variety of conditions. Several prior art dovetails have provided a ratio of neck width to a vertical bearing surface height of 1.19, 1.35 and 1.49. A prior art ratio of fan radius to vertical bearing surface height of 45, 47 and 56 has been provided, and a prior art ratio of dovetail width to vertical bearing surface height of 2.31, 2.93 and 2.99 has been provided.
Although geared architecture have improved propulsive efficiency, turbine engine manufacturers continue to seek further improvements to engine performance including improvements to thermal, transfer and propulsive efficiencies.